Fix memory alignment of outline spans.
Fix memory alignment of outline spans.

Some CPU architectures require all memory accesses to be naturally aligned.
We read polygon outline spans in pairs, so we need two underflow spans.

b/37478805

Change-Id: If74fd59654fb6fa56cbed93122b496b9e86dda06
Reviewed-on: https://swiftshader-review.googlesource.com/9528
Reviewed-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>
Tested-by: Nicolas Capens <0612353e1fcebbfd4b302d9337447ca0cdfa9122@google.com>

bkaradzic/SwiftShader,bkaradzic/SwiftShader,bkaradzic/SwiftShader,google/swiftshader,google/swiftshader,bkaradzic/SwiftShader,google/swiftshader,bkaradzic/SwiftShader